Oxide cathode for amplifier tubes



June 7, 1960 H. SCHMIDT-BRUCKEN 2,939,990

OXIDE CATHODE FOR AMPLIFIER TUBES Filed May 21, 1956 Fig. 1

WWW/WW Fig. 2

Control INVENTOR H. SCHMIDT-BRUCKEN ATTORNEY om CATHODE FOR AMPLIFIERTUBES Heinrich Schmidt-Briicken, Darmstadt, Germany, as-

signor to International Standard Electric Corporation, New York, N.Y., acorporation of Delaware Filed May 21, 1956, Ser. No. 586,045

12 Claims. (Cl. 313-346) This invention relates to improvements in orrelating to electric discharge tubes and more particularly to cathodesfor such tubes, of the oxide type and a novel manner of: controllingemission current therefrom. The emission current of the cathodes ofamplifier tubes can be controlled externally of the tube by means of thegrid voltage or the plate voltage. A change in the degree heating thecathode is not employed for control purposes due to the considerableinertia necessarily involved. Oxide cathodes are electrodes which havebeen coated with oxides of alkaline earth metals to improve electronemission at moderate temperatures. It is well-known that in oxidecathodes there are likely to be formed intermediate layers between thealkaline earth oxide and the core metal. These layers are the reactionproduct between the alkaline earth oxide and components resulting fromthe core metal. Thus it is known, for instance, that in the case ofnickel containing silicon there is formed a thin intermediate layer ofalkaline earth orthd silicates at the bottom of the alkaline earth oxidelayer. The formation of the intermediate layer is only effected on aheated cathode and very slowly, because the reacting component (e.g. theSi), only diffuses slowly out of the nickel and into thelayer ofalkaline earth oxide. This may take up to a thousand hours.

It is an object of this invention to provide a novel means to controlemission of a cathode by the interposition of an intermediate layerbetween the cathode support and the electron emissive coating.

It is a further object of the invention to control the emission of acathode by means of a barrier layer on said cathode.

The above-mentioned and other features and objects of this invention andthe manner of attaining them will become more apparent and the inventionitself will be best understood by reference to the following descriptionof an embodiment of the invention taken in conjunction with theaccompanying drawings, in which:

Fig. l is a schematic diagram of the cathode circuit of an amplifiertube;

Fig. 2 is a cross section of a cathode made in accordance with myinvention;

Fig. 3 is a cross section of another cathode comprising anotherembodiment of my invention;

Fig. 4 is a schematic diagram of a tube containing a cathode accordingto the invention; and

Fig. 5 is a schematic diagram of a circuit utilizing a tube containing acathode according to my invention, with input, output and modulatingcircuits.

The particular electrical effect of this intermediate layer is featuredby a deterioration of the conductivity of the emitting layer. Hence itacts like an electric resistance R inserted in the cathode lead. As iswell known, an un-bypassed resistance in series with a cathode producesa degenerative or negative current feedback, that is, it reduces theefiective transconductance of the tube. Since the intermediate layer isvery thin, the limiting surfaces thereof also have a noticeable capacitywith renited States Patent 0 2,939,990 Patented June 7, 1960 spect toone another, and constitute a by-pass condenser for resistance R bymeans of which a path for high hequencies is produced. In the equivalentcircuit diagram (Fig. 1), therefore, the resistance R inserted in thecathode lead is shown shunted by a condenser C The value of theresistance is of the order of about 40 ohms and [that of the layercapacity about 500 micromicrofarads, as regards 1 cm. surface. As hasbecome known recently, this intermediate layer does not represent acommon poorly-conductive layer such as an ohmic type resistance layer,but a barrier layer as exists in semiconductors with a rectifiercharacteristic.

The barrier layer action of this intermediate layer is successfullyutilized with the oxide cathode according to the invention. On thecathode carrier there is provided, in accordance with the invention, anintermediate layer serving to control the emission current. Hence in apredetermined manner there is produced an intermediate layer on asuitably arranged cathode carrier which layer is utilized forcontrolling the emission current. Thus, the cathode according to theinvention represents a tram sistor cathode, because the barrier layer,similar as in a transistor, is atfected by an ancillary electrode and isused for control purposes. In this case, however, unlike the normal typetransistor, there is not utilized and controlled the unidirectionalconductivity of the layer, but the emissive power of the cathode isalfected in a controllable manner. With the transistor there arerequired at least three electrodes in contact with the semiconductor.One of such electrodes is replaced in this case by the emitting surfaceof the cathode. The base metal of the cathode may act as the secondelectrode and for the third one an additional contact is introduced tothe alkaline earth oxide, that is, appropriately at a small distance(e.g. 50 from the base metal surface but, of course, without touchingthe metals.

However, the second and the third electrode may also be obtained bysplitting the base metal into two surfaces separated from each other,for example, in such a way that two thin Wires are double-wound closelynext to one another on an insulated heatable carrier (made for example,of ceramic material), but in such a Way that the two wires do not toucheach other. Thereafter the emitting layer would then have to be appliedover this arrangement of wires.

In order then to enable the formation of the intermediate layer,suitable wires would have to be employed. To this end there could beused eg. a wire of silicon-containing nickel, or also wires of purenickel which are coated with a silicon-containing lacquer. A materialknown under the trade name of Silikon, consisting ofa high polymersilicon-organic compound which is formed by groups of [R SiO] issuitable for the purpose. In this compound R is a radical hydrocarbonresidual, e.g. CH The formation of a barrier layer in the emissionsubstance near the metal wires can also be effected by means of asuitable previous treatment of the emitting layer itself. To this end,for instance, the cathode may be dipped during 'the preliminarytreatment into the aqueous solution of an electrolyte, and the wiresserving as base metal, may be connected during the electrolysis eitherindividually or in common as the cathode or the anode. By means of theions which are ten formed on the surface, the emitting layer surroundingthe wire may then be converted into an intermediate layer such asillustrated in Figs. 2 and 3. The single lead-in via the base metal ofthe cathode of a normal type of tube has thus been split up into atwo-terminal connection, so that the emission of the cathode can becontrolled by a voltage applied to these feeder terminals.

The subject matter of the invention will be described nickel wires.

more particularly inthefollowing with reference to the exemplifiedembodiments schematically showniuthefig: ures; of the accompanyingdrawings.

Fig. 2 shows a cross-section of a tube cathode having an'emitting: layerB arranged on the baser-metalt G;-

Withinthe emitting layer, but spaced a; small-1 distance di fromthe'surface of the base metal G, thereis: ar-

ranged the third electrode, Ni, which;is;formed egg. of

In this embodiment,- the first; electrode is constituted, bythe emittingsurface: and thefi second stituted by twothinwiresNiland NiZ made,. forinstance, of nickel, and which are; bifilar: wound: closely next to eachother onto the insulatedesupport K made of. ceramic material. In thise'rnbodiment;there= no necessity for spacing the; Wires: Niland:.from-.the surface of supportK.

There is shown in Fig. 4a, circuitidiagram ofia dischargetube utilizing.my novel cathode whereby the arrow P of the second lead-in denotes-.thebarrier-layer character of the arrangement, while the lead-in C formsthe; cathode conductor. is not:,shown in the drawing,,but:is presumed tolie beneath'the tube cathode. The employmentof such acontrollablecathode do not in-general? eliminate the needfor a. special controlgrid. In a tube according toiFig. 4 there will be -btained controlpossibilities via the control grid (or via a: plurality of. grids-),,as.well asalso viav the cathode. Hence the signal modulation may beapplied via the control grid, but, the amplitude level or' the dynamiclevel may, be controlled via'the cathode. .By utilizing my improvedcathode. the two control voltages arewidely shielded: from each other bythe sandwiched emitting layer. lying in betweenwithrespect toa mutualcapacitative interaction. Since by the control of: cathode emissionthetransconductance of. the-tube is modified, this arrangement lendsitself? suitably for. mixing or modulating purposes. The tube employingatransistor-cathode in: accordance with my invention, for example,could'be employedwithan arrangementzaccording to 'Fig. 5 where trollingmeans, said'tube comprising 'an envelope, cathode base means withinsaidenvelope, a layer of electron emissive material mountedon'said basemeans, a thin barrier having semiconductive properties locatedinterelectrode isconstituted by thebasemetal G; Ah'eating mediate saidbase means and said emissive material, means for causing electronemission from said emissive layer, and an emission-controlling conductordisposed in electrical contact with said thin barrier.

2. A: device as claimed in:claim:1,;whereinj said layer comprises earthalkaline oxides and saidbase means comprises an element made ofmetalcontaining silicon.

3. A device asrclaimedin claim- 2 wherein said emissioncontrollingconductors comprise a second element made of metal, spacedfromsaid;basemeans a predetermined distance.

4. A device as claimed'in'clainr l further comprising a non-conductivesupport for saidbase means.

5. A device as claimed in claim 4, wherein said base meansincludesffirst wires, saidcmissiomcontrolling,conductor comprisingsecond wires, and said two wires being wound about the surface ofsaid.non-conducting support inbifilar relationship. r

6. A deviceas' claimed in claim 5, wherein said wires are coated with alacquercontaining silicon.

As usual, the" heating filament in a grid input may be appliedthroughan-inpu't-trans- 'formerTi and the output of the tube may bederived via. To; both transformers being of conventionaltypes"andzhaving windings substantially matching theimpedances of thecircuits they interconnect. A voltage having a-frequency lower than thatapplied: to the grid may be'applied between ground and'the lead'P.

The invention as illustrated is shown applied to hard vacuum tubes, butit should be understood that the invention is broadlyapplicable to anyother type of tube where it is desirable to control electron emission,e.g., such asin gas-filled tubes.

. What is claimed is:

1. An electron tube having cathode emission con- 7. A device as claimedin clairnS, wherein said wires consist ofa nickel-silicon alloy.

8. A cathode as claimed in, claim 1, wherein said emission-controllingconductor comprises a Wire helix em: bedded in the layer.

9. A cathode as claimed in; claim 1, wherein said emission-controllingconductor is embedded within said layer.

10.- A circuit arrangement for a device as claimed in claim 1, furthercomprising a control gridandan anode, meansfor applying a signal of.first fiequency-to said control grid, means for applying a signalofsecondfrequency tosaid emission-controlling conductor, whereby saidfirst signal modulated by said second signal, will appear at said anode.

11. A. circuit arrangement. as in claim 10, wherein said" secondsignalhas aptrequency lowerthansaidfirst signal 12. A method ofproducing abarrierlayer ona cathfode for an electron discharge tubecomprisingthe steps of winding a pair ofwiresin abifilar. manner;aboutan electrically nonsconductive support; coveringsaidwires with alayer of, alkaline. earth oxide material, dipping the assembly into anaqueous solution of. antelectrolyte while applying. an electric. currentbetweenrsaid. pair. of wires, whereby ions are formed on the surface of'said Wires and a thin intermediate barrier layerv having.semiconductive properties is produced;

References Cited in the file of this patent UNITED STATES PATENTS2,121,589 Espe June 21, '1938 2,186,013 Edgerton V Y Jan. 9,- 19402,249,672 Spanner July 15, 1941 2,716,716v Hughes Aug.- 30, 19552,718,607 Katz -e Sept. 20, 1955 2,750,527 Katz a June 12, 19562,786,957 Huber Mar. 26, 1 957

1. AN ELECTRON TUBE HAVING CATHODE EMISSION CONTROLLING MEANS, SAID TUBECOMPRISING AN ENVELOPE, CATHODE BASE MEANS WITHIN SAID ENVELOPE, A LAYEROF ELECTRON EMISSIVE MATERIAL MOUNTED ON SAID BASE MEANS, A THIN BARRIERHAVING SEMICONDUCTIVE PROPERTIES LOCATED INTERMEDIATE SAID BASE MEANSAND SAID EMISSIVE MATERIAL, MEANS FOR CAUSING ELECTRON EMISSION FROMSAID EMISSIVE LAYER, AND AN EMISSION-CONTROLLING CONDUCTOR DISPOSED INELECTRICAL CONTACT WITH SAID THIN BARRIER.